تعریف و معنای "[read] {sb} like a book"

What is the origin of the idiom "read someone like a book" and when to use it?

The idiom "read someone like a book" likely comes from the idea that just as you can read the words in a book to understand its contents, you can "read" or understand a person by observing their actions, expressions, and behaviors. This expression is used to describe the ability to understand or perceive someone's thoughts, feelings, or motivations easily and accurately, often based on their behavior or body language.